The invention relates to a method and apparatus for discriminating between genuine and counterfeit documents, and in particular to sensing the complex electric permittivity characteristics of a document (e.g., paper currency, tickets, documents, etc) to determine its authenticity.
Genuine U.S. paper currency contains a variety of printed indicia which may be used to identify the currency as authentic, and also to distinguish between authentic currency of various denominations.
One indication of authenticity is the fact that certain areas on a U.S. bill are printed with ink with magnetic properties. For example, the portrait which appears in the center of every U.S. bill is, in a genuine bill, printed entirely with magnetic ink. The fanciful engraving which forms the printed border of each U.S. bill is likewise composed entirely of magnetic ink, as are the large capital letters or large numerals which appear to the right of the portrait and which identify the denomination of the bill (i.e., "ONE", "TWO", "FIVE", etc.). In contrast, the green Treasury Department seal that underlies the denomination identifying letters or numerals to the right of the portrait, as well as the black Federal Reserve Bank seal which appears to the left of the portrait, are both printed in non-magnetic ink
Each denomination U.S. bill is likewise characterized by the distance between the grid lines which comprise the background of the portrait field. In one dollar bills, for example, the space between vertical grid lines is equal to 0.008 inches. For two and five dollar bills, the grid line space is equal to 0.010 and 0.011 inches, respectively.
Prior art currency validators have been proposed that identify authentic U.S. bills and distinguish between bills of various denominations by measuring the average spacing between the vertical grid lines in the portrait areas of the bills. One such device is disclosed in U.S. Pat. No. 4,349,111 to Shah et al.
Identification of bills based on average grid line spacing is likely to lead to failures to distinguish between bills having relatively small differences in grid spacing. For example, certain commercial bill validators utilizing the average spacing technique cannot be used with both two dollar and five dollar bills, because the average grid line spacings are too similar.
Another problem with various prior art validators is that they may accept high denomination bills as valid lower denomination bills.
Many prior art current validators require that the tested bill be inserted into the validator in a specific orientation (e.g., Federal Reserve seal first). Such devices result in authentic bills being rejected merely because of improper orientation.
Many of the prior art currency validators require careful regulation of the speed at which the bill is scanned for information. In such validators, even a slight variation in scanning speed, such as that resulting from an instantaneous drop in power line voltage, can cause authentic bills to be rejected and produce inaccuracies in the identification of bill denomination. It is therefore desirable to provide a currency validator which is insensitive to the speed at which a bill is scanned.
In order to avoid some of the problems of speed regulation, some prior art validators, such as disclosed in U.S. Pat. No. 4,464,787 to Fish et al., employ detectors at fixed positions by positively identifying the position of the bill and thereby ascertain the bill area being tested. These validators, however, generally require a testing channel at least as long as the bill being tested.
In recent years, the high quality of copying and printing machines has resulted in a serious counterfeiting problem. One of the countermeasures against counterfeiting which has been adopted in many countries is the use of a metal embedded in paper checks or currency notes. However, metal detectors for automatically discriminating between genuine and counterfeit currency notes and checks by detecting the metal strip have not been put to practical use because they have not been capable of accurate and reliable detection of the metal strips at high speed.
An image (on a bill) can be scanned with a resolution of 300 or 600 dots per inch and the stored imager printed using inkjet printers that can match virtually any color.
The problem of counterfeiting is not limited to paper currency. Other documents such as checks, stock certificates, and airline and event tickets may also be subject to counterfeiting.
Therefore, there is a need to reliably verify the authenticity of various types of documents.